gpio: stm32: support disabling and reenabling interrupts on gpio pin

Up to now interrupts could be only configured once, with no way to
disable them in runtime.

Allow interrupts to be disabled in runtime and then properly reenabled
on user request. This allows to ignore interrupts when software is not
expecting them.

The improvement over previously reverted patch [1] is that we disable
interrupts only when we configure port for which interrupt line was
previously selected. This for example prevents to disable interrupts
line 2 in case PA2 was previously configured as interrupt source, but we
are currently configuring PB2 as output.

[1] 0951ce2d ("gpio: stm32: support disabling and reenabling
  interrupts on pin")

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>